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Nervous system

Nervous system. Functions of the nervous system. “Sense” the outside environment Sensory perception Also sense the inside environment Proprioception (knowing where aches are etc.) Integrate these sensory inputs Formulate a response to these sensory signals

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Nervous system

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  1. Nervous system

  2. Functions of the nervous system • “Sense” the outside environment • Sensory perception • Also sense the inside environment • Proprioception (knowing where aches are etc.) • Integrate these sensory inputs • Formulate a response to these sensory signals • Coordinate the response to these senses • Run from fear, eat when hungry, toilet when it’s popping out…

  3. Cells of the nervous system • Neuron: • Cell body = “soma” • Dendrites (reception) • Can a single dendrite, or many • Axon (send) • Can ONLY have 1 axon • Some neurons do not have an axon (axonemic) • Axons will “synapse” on various targets: • Other neurons • Muscle cells • Other internal organs • Cannot reproduce

  4. Cells of the nervous system • Neuroglia: • Support cells of the nervous system • Outnumber neurons 10:1 (10X as many neuroglial cells as there are neurons) • Act to protect, provide nutrition and help speed signal transmission • Can reproduce • Many different types of neuroglia • “glial” cells = a family of different cells that all serve to support neurons

  5. Cells of the nervous system • Neuroglia: • Astrocytes (“star”-shaped neuroglia) • Form the “blood-brain barrier” • Blood vessels enter the brain (the blood that enters the cranium is not filtered, but the blood vessels are very “tight” so that nutrients cannot leak out as easily) • Astrocytes line these blood vessels and “pull” out the nutrients and oxygen through active transport • Also help to pattern memory, filter cerebrospinal fluid and keep neurons “alive” by secreting hormone “nerve growth factor”

  6. http://www.fedem.org/revista/n1/imagenes/wekerle_fig5_peq.jpghttp://www.fedem.org/revista/n1/imagenes/wekerle_fig5_peq.jpg Astrocytes (red) and microglia cells (blue) Astrocytes (yellow) and microglia cells (green) http://www.angiochem.com/Section_images/images/Blood_18.jpg

  7. Cells of the nervous system http://neuromedia.neurobio.ucla.edu/campbell/nervous/wp_images%5C194_ependyma.gif • Neuroglia: • Ependymal cells (cube-shaped cells) • Line the ventricles of the brain and make cerebrospinal fluid (CSF) • Oligodendrocytes (oligodendrites) • Oligo = many, dendrite = tree • “arms” form individual myelin sheath • Oligo = forms many sheaths

  8. Cells of the nervous system • Neuroglia: • Microglia • Derived from the immune system • All other microglia & neurons arise from a single embryonic tissue type (neuroendoderm) • “resident” immune cells in the brain and spinal cord http://www.fedem.org/revista/n1/imagenes/wekerle_fig5_peq.jpg Microglia cells (green)

  9. Functions of neurons • Neurons are “single-minded” cells • Sensory: sends sensory information towards the central nervous system (“afferent”) • Gathers information and sends it to the brain and spinal cord • Motor: sends motor impulses to skeletal muscle or internal organs • “efferent” or “exiting” signals from the brain and spinal cord • Associative: associates/integrates the information • “interneurons” because they lie between the sensory and motor neurons

  10. Neuron vs. Nerve • Neuron = single cell • Nerve = bundle of neurons • Bundled similar to muscle • Keeping them bundled protects them • Can also cause some odd neural syndromes • Sensory impulses triggering motor impulses without the consent of the brain or spinal cord

  11. Myelination • Some neurons are myelinated, some are not • Myelination helps to speed the transduction of the signal • Nearly 10-100X faster signal speed vs. non-myelinated neuron • Remember oligodendrites? • In the brain & spinal cord, white matter = myelinated • Oligodendrites form individual myelin sheaths • Only the axon is myelinated (never the dendrites) • Outside the brain & spinal cord, myelin sheath is formed by Schwann cells

  12. Myelination

  13. Neural “excitability” • Nerves are excitable (just like muscle cells) • Neurons can respond to many stimuli • Muscle cells can ONLY respond to neurons • Excitability is dependant upon the electrochemical gradient that exists across the neuron plasma membrane (neurolemma) • Electrical gradient = more positive (+) charges outside the cell • Chemical gradient = more sodium ions outside, more potassium ions inside • The Na+/K+/ATPase enzyme creates the conditions for excitability (does not participate in the actual “excitation”)

  14. Resting membrane potential: the amount of potential energy (in the form of volts) that is “stored” within a cell. Outside/extracellular fluid This electro (+) and chemical (Na+ outside, K+ inside) is created by the sodium-potassium ATPase enzyme(Na+/K+/ATPase). For 1 ATP molecule, this protein (it acts as a transporter) pumps OUT 3 sodium ions (Na+), and brings IN 2 potassium ions (K+). The net effect is to create a chemical gradient of sodium and potassium that is also an electrical gradient (3:2). (+ + + + +) (- - - - -) Inside/intracellular fluid

  15. Neural “excitability” • In order to excite a neuron (to fire an action potential) • Neuron will sense something • Each neuron senses a particular target (are very specific in what they can respond to) • Another neuron, heat, light, tactile (feel) etc. • Neuron “senses” with receptors • Receptors are specific for their target • Receptors act as sodium channels when triggered • Remember that a receptor is a protein • It has an “active site” similar to an enzyme…it will only “sense” or receive a SPECIFIC signal

  16. Neural “excitability” • When a neural receptor is excited (is triggered by a target stimulus) • Will normally then open up a pore that allows sodium to pass • Sodium ions outside the cell will rush into the cell • Sodium enters via concentration gradient • Na+ ions carry a (+) charge • Brings in (+) charges into the cell (decreases the electrical gradient)

  17. Na+ ions outside K+ K+ K+ K+ K+ K+ K+ K+ ions inside

  18. Neural “excitability” • Neuron adds up all the excitable stimuli it receives • If there is enough, it will then send a message down its axon to the target recipient • Action potential is the “signal” pulse that a neuron sends • All-or-none response (100% or nothing at all) • Note: not all stimuli will result in an action potential • Must be strong enough • Must be “numeric” enough

  19. Neural “excitability” • Along the axon, an action potential “carried” or propagated by a slightly different mechanism • Instead of receptors, “voltage-sensitive” sodium channels are used • Respond to voltage across the plasma membrane, NOT to stimuli like heat, light etc. • Provides the all-or-none response • As long as the membrane is depolarized enough, they will open

  20. Na+ ions outside K+ K+ K+ K+ K+ K+ K+ K+ ions inside

  21. Na+ ions outside K+ K+ K+ K+ K+ K+ K+ K+ ions inside

  22. Na+ ions outside K+ K+ K+ K+ K+ K+ K+

  23. Depolarization: when you deplete the membrane potential (like using up a battery). As the battery “dies” the membrane potential weakens. Outside/extracellular fluid 0 (+ + + + + + +) (+ + + +) Vs. Vs. (- - - - - - - - - - ) (- - - -) 0 Inside/intracellular fluid

  24. Neural “excitability” • Action potential will then travel down the axon to the terminal portion (synapse) • At the synapse, action potential triggers the release of “neurotransmitter” • Neuron can only make 1 neurotransmitter • Stored in “vesicles” that are docked or “pre-fused” to the plasma membrane • Over 100 different neurotransmitters • Neurotransmitter released by exocytosis • Neurotransmitter diffuses to the adjacent target cell (another neuron, muscle cell or organ)

  25. Neural “excitability” • Neurotransmitter then binds to specific neurotransmitter receptor • Just like the original “stimulus” receptor that started this whole process • On another neuron, the process will start again (open an ion channel, stimulate etc.) • On skeletal muscle, will trigger a similar process • On an organ, will usually trigger chemical changes within the cell (second messenger) • Usually not an ion channel

  26. Voltage-gated receptors: will open when they sense a change in the voltage across the inside/outside of the neurolemma. Once open, they allow sodium ions to rush into the cell. Ligand-gated receptors: will open when they receive the correct stimulation. Once open, they allow sodium ions to rush into the cell.

  27. Neural “excitability” • Need to re-polarize the membrane after the stimulus or action potential • Remember that the membrane electrical and chemical gradients are now diffused or discharged because the sodium ions have rushed into the cell • Also remember that the cell still retains a great deal of potassium ions within the cell • Thus, if you open a channel to permit potassium to rush out, those (+) ions will leave, and return the electrical gradient

  28. K+ K+ K+ K+ K+ K+ K+ K+ K+

  29. (+ + + + + + +) (- - - - - - - - - - ) Repolarization: When you re-charge the membrane potential to the original “strength”. Outside/extracellular fluid (0) Vs. (0) Inside/intracellular fluid

  30. In order to re-polarize the membrane, you need to move ions (sequester or push them to one side of the lipid bilayer) Outside/extracellular fluid (+ +) To re-charge the membrane potential, you need to push out (+) charges (in other words, you need to sequester more (+) charges outside the cell than are on the inside) (- -) Inside/intracellular fluid

  31. In order to re-polarize the membrane, you need to move ions (sequester them on one side of the lipid bilayer) Outside/extracellular fluid • You can re-charge the membrane potential by: • Using the Na+/K+/ATPase • Opening a potassium channel (remember the cell stores more K+ inside than is outside) (+ +) (- -) Inside/intracellular fluid

  32. (+ +) (- -) The fastest way to re-charge membrane potential is to allow potassium to rush out of the cell by opening a potassium channel. Outside/extracellular fluid By opening a potassium channel, you allow all the stored K+ to rush out quickly, just like if you were to let the air out of a tire. Inside/intracellular fluid

  33. (+ + + + + + +) Resting (- - - - - - - - - - ) K+ K+ K+ K+ K+ K+ K+ Notice how simply moving potassium ions can re-charge the electrical gradient. However, also notice the reversal of the chemical gradient. K+ K+ K+ K+ K+ (+ + + + + + +) K+ K+ Recharge / repolarization (- - - - - - - - - - ) K+ K+

  34. (+ + + + + + +) (- - - - - - - - - - ) Once you restore the membrane potential (repolarize the membrane), you use the Na+/K+/ATPase to restore the chemical gradient while maintaining the electrical gradient Outside/extracellular fluid Inside/intracellular fluid

  35. Myelination • Recall that myelination helps speed the action potential • The depolarizations “jump” from node to node (the jump across each sheath) • “saltatory conduction” = “jumping” conduction

  36. Unmyelinated axon Action potential gets this far in 10 seconds, vs Node 1 Myelin sheath Node 2 Action potential gets this far in 11 seconds

  37. On the outside of the axon, you only see sodium ions (all the potassium ions are sequestered within the axon) Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath Na+ ions outside

  38. INSIDE the axon, you can see all the potassium ions (grey), and a few sodium ions as well (cells usually have SOME sodium, just not very much) Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ Na+ ions outside

  39. When you trigger an action potential, you allow sodium to flood INTO the axon Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ Na+ ions outside

  40. As the sodium concentrates with the region of the node of Ranvier, it repels sodium and potassium ions underneath the myelin sheath Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ K+ Na+ ions outside

  41. The “push” of sodium ions into the axon then results in more “+” charges (sodium and potassium) in the next node of Ranvier Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ K+ Na+ ions outside

  42. The presence of additional “+” charges (as sodium and potassium ions) underneath the axolemma of the next node of Ranvier results in a decrease in membrane potential…this stimulates the voltage-regulated sodium channels in this region to open up. Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ K+ Na+ ions outside

  43. Remember that once the membrane potential reaches a particular point, the voltage-regulate sodium channels will close, and the potassium channels will open, to allow some of the potassium ions to leak out (they’ll want to leave because all the Na+ ions are electrically repelling them) Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ ions “leaving” the axon

  44. Since the potassium ions are leaving the axon, the left node of Ranvier is said to be in the “refractory period” and cannot trigger another action potential until the correct membrane potential is restored (restoration of the membrane potential is carried out by pushing out potassium ions). This is why the action potential can only proceed in 1 direction. Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ ions “leaving” the axon

  45. The process of opening voltage-regulated sodium channels, and “pushing” the “+” charges to the next node of Ranvier occurs so fast that it is like an unmyelinated neuron…just with a space or gap between action potential. Axon (node of Ranvier) Axon (node of Ranvier) Myelin sheath K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ ions “leaving” the axon

  46. Neurotransmitters • Remember that neurons do not physically “touch” other cells • Communicate by release of neurotransmitters • Recall that 1 neuron can only produce 1 type of neurotransmitter

  47. Neurotransmitters • Over 100+ chemicals/proteins can act as a neurotransmitter • Generally grouped into 4 categories: • Acetylcholine: acetic acid and choline (amino acid) • Amino acids: glycine, glutamate, aspartate, and GABA (-amino-butyric-acid) • Monoamines: amino acid without the carboxyl (COOH-) • Only have the amino group attached…mono-amine • Epinephrine, norepinephrine, dopamine (catecholamines) • Histamine, serotonin (5-hydroxy trypamine…5-HT) • Neuropeptides: 2-4 amino acid peptides • -endorphin, Substance-P • Normally have greater effects with less concentration, and last longer • Stored in secretory granules rather than synaptic vesicles (because they are physically larger) • MANY are made in the intestine and then sent to the brain = “gut-brain peptides”

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